1. Field of the Invention
The present invention is broadly concerned with respiratory apparatus of the type commonly used by commercial or private aircraft crew during emergency situations. More particularly, the invention pertains to such respiratory apparatus having a full or partial face mask with an extensible and inflatable strap element coupled to the mask which is initially inflated and expanded to allow ready donning of the respiratory device followed by deflation of the strap element to draw the mask into tight engagement with the user's head; the respiratory apparatus of the invention includes a comfort control assembly permitting controlled lessening of the engagement force exerted by the mask against the user's face so that the apparatus may be worn for an extended period of time without discomfort.
2. Description of the Prior Art
An inflatable head harness for respirator devices is described and illustrated in U.S. Pat. No. 3,599,636 and comprises a mask that is connected to an elongated, extensible harness or strap having internal conduits connected by a valve to a source of pressurized air. When the valve is opened, air admitted to the conduits of the strap cause the strap to stretch and assume a somewhat rigid configuration. In this manner, the user can grasp the mask with one hand and direct the inflated strap behind his or her head, a particularly useful feature in an emergency situation for a flight crew when only one free hand is available.
Once the harness of the respirator shown in U.S. Pat. No. 3,599,636 is placed over the head, the strap is deflated and contracts in length. Thereafter, the inherent resiliency of the deflated strap urges the mask in tight engagement with the nose and mouth areas of the wearer's face in an attempt to avoid peripheral leakage of the breathable gas.
As a rule, flight crew masks must be pressurized when the aircraft is flying at cabin altitudes above approximately 40,000 feet in order to force air into the user's lungs. At these altitudes, therefore, the straps must exert a relatively large biasing force pressing the mask against the face to overcome the pressure of the oxygen urging the mask away from the skin and prevent oxygen leakage around the peripheral seal of the mask. However, at cabin altitudes of less then 40,000 feet, pressurized breathing conditions within the chamber of the mask are unnecessary and the regulator operates upon demand breathing such that an oxygen enriched air mixture is admitted to the mask only as the user inhales.
In general, the substantial majority of flight time is incurred at cabin altitudes at less then 40,000 feet. There are many situations, however, where the respiratory mask must be worn at all times such as cases where only one crew member is present. Therefore, the harness straps represent a substantial source of discomfort at lower altitudes when the respirator must be worn on the head at all times since the straps normally present a large degree of force even though pressurized breathing conditions are unnecessary.
U.S. Pat. No. 5,036,846 describes an inflatable harness crew oxygen mask provided with a pneumatic comfort adjustment. In the '846 patent, inflation control means is provided having structure for selective establishing and maintaining the inflatable strap element at an intermediate pressure therein between the high-pressure extended position of the strap element and low-pressure retracted position thereof. Similarly, U.S. Pat. Nos. 5,623,923 and 5,503,147 describe inflatable strap comfort control devices which selectively inflate or deflate the strap element during use so as to achieve user comfort.
A problem with these prior comfort adjustment devices stems from the pneumatic character thereof i.e., they rely upon controlling pressure conditions within the strap element. However, this can be a problem if the strap element experiences significant leakage, in as much as the comfort control feature can then be rendered inoperative.
There is accordingly a need in the art an improved comfort control assembly forming a part of an inflatable harness respiratory device which permits comfort control without the need for controlled partial inflation or deflation of the strap element during wearing of the respiratory device. Preferably, such an improved comfort assembly would permit the desired degree of comfort control adjustment while the strap element remains fully deflated and essentially at ambient pressure.